[OPTIM] session: don't recheck analysers when buffer flags have not changed

[OPTIM] session: don't recheck analysers when buffer flags have not changed

Analysers were re-evaluated when some flags were still present in the
buffers, even if they had not changed since previous pass, resulting
in a waste of CPU cycles.

Ensuring that the flags have changed has saved some useless calls :

  function            min calls per session (before -> after)

  http_request_forward_body       5 -> 4
  http_response_forward_body      3 -> 2
  http_sync_req_state            10 -> 8
  http_sync_res_state             8 -> 6
  http_resync_states              8 -> 6